Lyme disease is the most common arthropod-borne infection in the United States. Over 100,000 cases have been reported to date. The disease is a multisystem disorder with dermatologic, neurologic and rheumatologic manifestations. Borrelia burgdorferi, the causative agent of Lyme disease, alternates between the microenvironments of the tick vector and a mammalian host. The environmental conditions the spirochete encounters during its infectious cycle would suggest the ability to alter transcription and protein expression in response to environmental cues such as temperature and pH. In this study we identify alterations in the membrane protein profile, as determined by immunoblotting and two-dimensional, pH gradient gel electrophoresis, that occur in virulent B. burgdorferi as the pH of the growth medium is altered. Initial comparisons of cultures incubated at pH?s 6.0, 7.0 and 8.0 yielded alterations in the expression of seven membrane proteins. Six of these membrane proteins were present in increased amounts in or solely expressed by cultures incubated at pH?s 6.0 and 7.0. The 24-kDa protein that decreased in expression at pH 8.0 was identified as outer surface protein C (OspC). In addition, a 42-kDa membrane protein increased in amounts in cultures incubated at pH 8.0. Similar changes were observed with serum from a mouse infected by tick bite, with the recognition of two additional proteins unique to pH?s 6.0 and 7.0. When membrane fractions were analyzed by 2D-pH gradient gel electrophoresis, at least 37 changes in the membrane protein profile between cells incubated at pH?s 6.0, 7.0 and 8.0 were observed. Environmental cues such as pH may prove important in the regulation of virulence determinants and factors necessary for the adaptation on B. burgdorferi to the tick or mammalian microcosm. - Lyme disease, Borrelia burgdorferi, membrane proteins, pH regulated proteins, 2D gel electrophoresis